Waterbed or liquid filled mattresses are very flexible, particularly at the edges and at the corners of the mattress, and it is somewhat difficult to cause a sheet to stay in place on the waterbed mattress. Typically, even a form fitting bottom sheet that fits about the side and end edges of the waterbed mattress can be dislodged in normal use of the waterbed. Further, the attachment of the top sheet at the foot of the waterbed is even more difficult, and many top and bottom sheets for waterbeds are made as a unitary structure by being connected together along their bottom edges. This allows the top sheet to be secured to the bed by the more securely attached bottom, form fitting sheet that extends about the corner structures of the waterbed mattress. However, when the top sheet is pulled during normal use of the bed, there is some likelihood that both the top and bottom sheets will be pulled off the foot of the mattress.
One of the prior art waterbed sheet arrangements is disclosed in U.S. Pat. No. 3,996,633, whereby corner pockets are formed in the bottom sheet so as to fit about and partially beneath the corners of the waterbed mattress. However, the corner structures of waterbed mattresses are so flexible that the form fitting sheets especially designed and precisely formed to tightly fit the corners of a waterbed mattress still can be inadvertantly dislodged during use of the bed.